The Li/Li.sub.x CoO.sub.2 cell attracts attention as a result of its high voltage and high energy density. However its high OCV and charging voltage cause eletrolyte oxidation. This problem is more severe in the Li/Li.sub.x CoO.sub.2 system where X is about 0.4-1.0 than in some other rechargeable lithium cells. Recently, LiAsF.sub.6 in MF+DEC (or DMC) was suggested as an electrolyte for this cell. It was found that the presence of DEC and DMC increases the oxidation potential of the electrolyte. Cells with LiAsF-MF-DMC (or DEC) electrolye exhibited long cycle life but a relatively fast self-discharge.
At low x values Li.sub.x CoO.sub.2 is thermally unstable, below 0.3, and decomposes to Li.sub.1 CoO.sub.2 and cobalt oxide. The intercalation of lithium into the Li.sub.x CoO.sub.2 crystal causes a decrease in layer spacings. This may decrease the diffusion coefficient of lithium in Li.sub.x CoO.sub.2 for x values near unity.
It was hypothesized that the incorporation of a doubly charged but larger cation, such as Ca.sup.++, Sr.sup.++ or Ba.sup.++ into the Li.sub.x CoO.sub.2 crystal lattice may increase the lattice spacings on one hand and stabilize it on the other.
The degradation modes of rechargeable (and also for primary lithium batteries are: (1) Reduction of the solvents and electrolyte salts at the anode (lithium, lithium alloy or lithium intercalated carbonaceous material). (2) Oxidation of the solvents or lithium salts at the cathode. (3) Degradation of the electrolyte due to polymerization reactions, hydrolysis reactions and other salt-solvent reactions. All these degradation processes are accelerated by impurities such as water and acids. Addition of desiccant and buffers which can absorb water and neutralize acids may thus increase the service life of these lithium batteries.
From the prior art there are known cathode materials which are of the formula corresponding to a mixture of Li.sub.2 CO.sub.3 +Co.sub.2 O.sub.3, which are prepared by admixing the components and subjecting them to a high temperature.
EDA (U.S. Pat. No. 4,336,315) relates to a cell with a light metal anode, a cathode with manganese oxide as active material which also contains an alkaline earth oxide, and an organic electrolyte. As set out in column 2, lines 16 to 29, the alkaline earth metal oxide as solid base is admixed with the manganese dioxide to neutralize and inactivate the solid acidity, based on the surface hydroxyl groups of the manganese dioxide.
The cell of the '315 patent is a primary cell which cannot be recharged, and in this differs basically from the rechargable cells of the present invention.
Furthermore, the addition of an alkaline earth oxide serves to neutralize solid activity, and is in the form of an admixture and not incorporated into a crystalline compound. It is heated to 400.degree. C. (column 3, line 56) for dehydration, by itself.
The EDA cell ought to be compared with the cells of the invention, and especially with FIG. 5 which indicates possible number of efficient cycles of the novel cell.
FONG (U.S. Pat. No. 5,028,500) relates to a carbonaceous electrodes for lithium cells, and the emphasis is on particulate carbonaceous electrodes intercalable with the alkali metal. A further emphasis is on the degree of graphitization of the carbon.
SUGENO (U.S. Pat. No. 5,168,019) relates to a secondary battery which contains an electrolyte to which there is added an oxide selected from Al.sub.2 O.sub.3, BaO.sub.2 and MgO, and where the cathode is made from a carbon material which may be doped with lithium, and where the anode is a composite oxide of lithium and cobalt.
This patent has nothing in common with the specific crystalline materials which are the material of the cathodes of the cells of the invention.